# Debian Networking

Configure and troubleshoot network interfaces, routing, and DNS on Debian.

## Network Interfaces

Show interfaces and addresses:

```bash
ip addr show
ip addr show eth0
```

Bring interface up/down:

```bash
sudo ip link set eth0 up
sudo ip link set eth0 down
```

Assign IP address (temporary):

```bash
sudo ip addr add 192.168.1.100/24 dev eth0
sudo ip addr del 192.168.1.100/24 dev eth0
```

## Routing

Show routing table:

```bash
ip route show
```

Add default gateway:

```bash
sudo ip route add default via 192.168.1.1
```

Add static route:

```bash
sudo ip route add 10.0.0.0/8 via 192.168.1.1
```

## DNS

Show DNS servers:

```bash
cat /etc/resolv.conf
```

Set temporarily:

```bash
echo "nameserver 8.8.8.8" | sudo tee /etc/resolv.conf
```

Persistent via `/etc/network/interfaces`:

```
auto eth0
iface eth0 inet dhcp
    dns-nameservers 8.8.8.8 8.8.4.4
```

Or use NetworkManager (GUI preferred on desktop):

```bash
nmcli device show eth0
nmcli connection modify eth0 ipv4.dns "8.8.8.8 8.8.4.4"
```

## Testing Connectivity

Ping a host:

```bash
ping -c 4 8.8.8.8
```

Trace route to host:

```bash
traceroute google.com
```

Test DNS:

```bash
nslookup google.com
dig google.com
```

Check open ports:

```bash
ss -tlnp         # listening TCP ports
ss -ulnp         # listening UDP ports
netstat -tlnp    # old alternative
```

## Hostname

Show current hostname:

```bash
hostname
```

Set permanently:

```bash
sudo hostnamectl set-hostname newname
```

Or edit directly:

```bash
sudo nano /etc/hostname
sudo systemctl restart systemd-hostnamed
```

## Static Configuration

Edit `/etc/network/interfaces`:

```
auto eth0
iface eth0 inet static
    address 192.168.1.100
    netmask 255.255.255.0
    gateway 192.168.1.1
    dns-nameservers 8.8.8.8
```

Apply changes:

```bash
sudo systemctl restart networking
```

DHCP is simpler for most systems; use static only when necessary.